The present invention broadly relates to a valve assembly for use immersed in a liquid metal. It is particularly directed to a check valve assembly for interconnecting a pump discharge duct and a reactor coolant inlet duct in a pool-type, liquid-metal cooled, nuclear reactor.
The design of check valves and conduit assemblies for use in pool-type reactors is very demanding since there is a need for minimizing any stress resulting from thermal expansion and relative movement of the two components being interconnected. In addition, they must be capable of withstanding the calculated stress which would result from a seismic event having a specified magnitude. The problem in the design of check valve assemblies is further complicated by the fact that the valve assembly is generally inaccessible for inservice inspection since it is submerged in a pool of liquid metal.
In the design of nuclear reactors it is customary to provide a certain amount of redundancy in the interest of safety. For example, a loss of coolant accident could be catastrophic since the core would continue to generate heat and could result in a melt-down of the individual fuel assemblies. Accordingly, designs for such reactors usually provide for two or more pumps for supplying coolant through the reactor core. The use of multiple pumps, however, does present an additional problem. Specifically, if one pump fails coolant will flow not only through the reactor core from the other pump, but also will flow in a direction back through the inoperative pump. Accordingly, to offset this effect, some additional provisions must be made. It has been proposed to design the pumps with sufficient excess capacity to provide the required coolant flow through the reactor core as well as that which would backflow through the inoperative pump. This procedure results in a requirement for larger pumps and an increase in operating costs.
Another proposed approach utilizes a check valve located between the outlet of the pump and the inlet to the reactor core. This effectively eliminates backflow through the pump and the necessity of having pumps sized to accommodate such backflow. In the case of a pool-type reactor, however, this necessarily requires that the check valve be located within the pool of coolant. Since the check valve involves moving parts it should be accessible for inspection, maintenance and repair if required. To provide such access it has been proposed to locate the check valve in a well or tank which extends from an upper portion of the reactor vessel down into the pool of liquid coolant. The tank and check valve are connected with nonremovable piping to the outlet of the pump and the inlet to the reactor core. A disadvantage of this approach was the necessity of an additional penetration through the reactor vessel, the space required for the tank and the increased cost. In addition, this approach could only be used with nonremovable piping.
Obviously it would be advantageous to have a check valve assembly which did not require a separate penetration through the reactor vessel since such penetration potentially jeopardizes the integrity of the vessel. In addition, it would be advantageous as such if a check valve assembly could be provided which could be utilized with removable piping.